In a typical radio communications network, wireless terminals, also known as mobile stations, terminals and/or user equipments, UEs, communicate via a Radio Access Network, RAN, to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station or network node, e.g. a radio base station, RBS, which in some networks may also be referred to as, for example, “NodeB”, “eNB” or “eNodeB”.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, for user equipments. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations/network nodes connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially flat architecture comprising radio base station nodes without reporting to RNCs.
Random Access
In LTE, as in any communication system, a wireless communication device may need to contact the network, via the base station (eNodeB), without having a dedicated resource in the uplink, UL, i.e. from a wireless communication device to a base station. To handle this, a random access procedure is available where a wireless communication device that does not have a dedicated UL resource may transmit a signal to the base station. The first message of this procedure is typically transmitted on a special resource reserved for random access, a physical random access channel, PRACH. This channel may for instance be limited in time and/or frequency, as in LTE. This is illustrated in FIG. 1. The resources available for PRACH transmission is provided to the terminals as part of the broadcasted system information in system information block 2 (SIB-2) (or as part of dedicated RRC signaling in case of e.g. handover).
The resources consist of a preamble sequence and a time/frequency resource. In each cell, there are 64 preamble sequences available. Two subsets of the 64 sequences are defined, where the set of sequences in each subset is signaled as part of the system information. When performing a (contention-based) random-access attempt, the terminal selects at random one sequence in one of the subsets. As long as no other terminal is performing a random-access attempt using the same sequence at the same time instant, no collisions will occur and the attempt will, with a high likelihood, be detected by the eNodeB.
In LTE, the random access procedure may be used for a number of different reasons. Among these reasons are, for example: initial access (for wireless communication devices in the RRC_IDLE state), incoming handover, resynchronization of the UL, scheduling request (for a wireless communication device that is not allocated any other resource for contacting the base station), positioning, etc.
FIG. 2 illustrates the contention-based random access procedure used in LTE Rel-10, i.e. shows signalling over the air interface for the contention-based random access procedure in LTE.
The wireless communication device starts the random access procedure by randomly selecting one of the preambles available for contention-based random access. The wireless communication device then transmits the selected random access preamble on the physical random access channel (PRACH) to eNodeB in RAN. RACH is a transport channel which is transmitted by the wireless communication device over PRACH.
The RAN acknowledges any preamble it detects by transmitting a random access response (MSG2) including an initial grant to be used on the uplink shared channel, a temporary C-RNTI (TC-RNTI), and a time alignment (TA) update based on the timing offset of the preamble measured by the eNodeB on the PRACH. The MSG2 is transmitted in the DL to the wireless communication device using the PDSCH and its corresponding PDCCH message that schedules the PDSCH contains a cyclic redundancy check (CRC) which is scrambled with the RA-RNTI. When receiving the response the wireless communication device uses the grant to transmit a message (MSG3) that in part is used to trigger the establishment of radio resource control and in part to uniquely identify the wireless communication device on the common channels of the cell. The timing alignment command provided in the random access response is applied in the UL transmission in MSG3. In addition, the eNB may also change the resources blocks that are assigned for a MSG3 transmission by sending an UL grant that has its CRC scrambled with the TC-RNTI which was included in MSG2. In this case the PDCCH is used, to transmit the DCI containing the uplink grant. The MSG4, which is then contention resolving, has its PDCCH CRC scrambled with the C-RNTI if the wireless communication device previously has a C-RNTI assigned. If the wireless communication device does not have a C-RNTI previously assigned, it has its PDCCH CRC is scrambled with the TC-RNTI obtained from MSG2. The procedure ends with RAN solving any preamble contention that may have occurred for the case that multiple wireless communication devices transmitted the same preamble at the same time. This may occur since each wireless communication device randomly selects when to transmit and which preamble to use. If multiple wireless communication devices select the same preamble for the transmission on RACH, there will be contention between these wireless communication devices that needs to be resolved through the contention resolution message (MSG4).
The case when contention occurs is illustrated in FIG. 3. FIG. 3 illustrates contention based random access, where there is contention between two wireless communication devices. Here, two wireless communication devices transmit the same preamble, p5, at the same time. A third wireless communication device also transmits at the same time and the same RACH, but since it transmits with a different preamble, p1, there is no contention between this wireless communication device and the other two wireless communication devices.
The wireless communication device may also perform non-contention based random access. A non-contention based random access or contention free random access may e.g. be initiated by the eNB to get the wireless communication device to achieve synchronisation in UL. The eNB initiates a non-contention based random access either by sending a PDCCH order or indicating it in an RRC message. The later of the two is used in case of HO.
The eNB may also order the wireless communication device through a PDCCH message to perform a contention based random access; the procedure for this is illustrated in FIG. 3. The procedure for the wireless communication device to perform contention free random access is illustrated below in FIG. 4. Similar to the contention based random access the MSG2 is transmitted in the DL to the wireless communication device and its corresponding PDCCH message CRC is scrambled with the RA-RNTI. The wireless communication device considers the contention resolution successfully completed after it has received MSG2 successfully. For the contention free random access as for the contention based random access does the MSG2 contain a timing alignment value. This enables the eNB to set the initial/updated timing according to the wireless communication devices transmitted preamble.
FIG. 4 shows signalling over the air interface for the contention-free random access procedure in LTE.
Dual Connectivity
A dual connectivity framework is currently being considered for LTE Rel-12. Dual Connectivity, DC, refers to the operation where a given wireless communication device consumes radio resources provided by at least two different network points, i.e. a Master eNB, MeNB, and a Secondary eNB, SeNB, connected with non-ideal backhaul while in RRC_CONNECTED. A wireless communication device in dual connectivity maintains simultaneous connections to anchor and booster nodes, where the MeNB is interchangeably called an anchor node is and the SeNB is interchangeably called a booster node. As the name implies, the MeNB controls the connection and handover of SeNB. No SeNB standalone handover is defined for Rel-12. Signaling in MeNB is needed even in SeNB change. Both the anchor node and booster node may terminate the control plane connection towards the wireless communication device and may thus be the controlling nodes of the wireless communication device.
The wireless communication device reads system information from the anchor node. In addition to the anchor node, the wireless communication device may be connected to one or several booster nodes for added user plane support. The MeNB and SeNB are connected via the Xn interface, which is currently selected to be the same as the X2 interface between two eNBs.
More specifically dual connectivity (DC) is a mode of operation of a wireless communication device in RRC_CONNECTED state, where the wireless communication device is configured with a Master Cell Group (MCG) and a Secondary Cell Group (SCG). Cell Group (CG) is a group of serving cells associated with either the MeNB or the SeNB. The MCG and SCG are defined as follows:                Master Cell Group (MCG) is a group of serving cells associated with the MeNB, comprising of the PCell and optionally one or more SCells.        Secondary Cell Group (SCG) is a group of serving cells associated with the SeNB comprising of PSCell (Primary SCell) and optionally one or more SCells        
Master eNB is the eNB which terminates at least S1-MME. Secondary eNB is the eNB that is providing additional radio resources for the wireless communication device but is not the Master eNB.
FIG. 5 illustrates a dual connectivity deployment scenario. Here, a dual connectivity setup is described. In this example, only one SeNB is connected to wireless communication device, however, more than one SeNB may serve the wireless communication device in general. As shown in FIG. 5, it is also clear that dual connectivity is a specific feature of the wireless communication device and a network node may support a dual connected wireless communication device and a legacy wireless communication device at the same time. As mentioned earlier, the anchor and booster roles are defined from a point of view of the wireless communication device. This means that a node that acts as an anchor to one wireless communication device may act as booster to another wireless communication device. Similarly, though the wireless communication device reads the system information from the anchor node, a node acting as a booster to one wireless communication device, may or may not distribute system information to another wireless communication device. It is worth mentioning here that, we have used anchor node and MeNB with interchangeable meaning, similarly, SeNB and booster node is also used interchangeably in the document.
MeNB may provide system information, terminate control plane and may also terminate user plane. SeNB may terminate control plane and may also terminate only user plane.
In one implementation, dual connectivity allows a wireless communication device to be connected to two nodes to receive data from both nodes to increase its data rate. This user plane aggregation achieves similar benefits as carrier aggregation using network nodes that are not connected by low-latency backhaul/network connection, also referred to as an ideal backhaul. Due to this lack of low-latency backhaul, the scheduling and HARQ-ACK feedback from the wireless communication device to each of the nodes will need to be performed separately. That is, it is expected that the wireless communication device shall have two UL transmitters to transmit UL control and data to the connected nodes.
PSCell Activation in Dual Connectivity
In dual connectivity the wireless communication device is connected to two eNodeBs simultaneously, i.e. to MeNB and SeNB. Each of them may have one or more associated SCells which may be configured for downlink (DL) Carrier Aggregation (CA) operation, or downlink (DL) and uplink (UL) CA operation. The SCells are time-aligned to the MeNB and SeNB, respectively, but the MeNB and SeNB may or may not be time aligned with respect to frame timing and/or Serial Frame Number, SFN. Two modes of operation are defined:                Synchronized operation, where the downlink frame time difference between PCell and PSCell is within ±33 us, and        Unsynchronized operation, where the time difference between PCell and PSCell is arbitrary but limited to 0.5 ms.        
With respect the PSCell it has been agreed that the PSCell is configured by the PCell (i.e. by MeNB), and that the PSCell is activated at the configuration and cannot be deactivated by MeNB or SeNB. Configuration and simultaneous activation of PSCell is done by MeNB, but otherwise the MeNB and SeNB operate the wireless communication device independently. Particularly, when the wireless communication device gets the SeNB activated it first has to carry out random access towards PSCell to establish a connection and get allocations so that it may send a first CQI report indicating the quality of the link as well as confirming that the activation has been successful.